1. Field of the Invention
The present invention relates to a method for preparing composite materials of a positive temperature coefficient thermistor, and more particularly, to a method for preparing a conductive polymeric composite material having carbon black utilized to a structure for composite materials of a positive temperature coefficient thermistor.
2. Description of Related Art
Thermistor devices are already widely used in many fields, such as temperature detection, security control, temperature compensation, and so on. In the past, a thermistor device has mainly utilized ceramic material. However, ceramic material needs to be manufactured at high temperatures, in most cases, higher than 900° C. Thus the energy consumption is enormous, and the process is very complicated. Later on, a thermistor device utilizing a polymeric substrate is developed. Since the manufacturing temperature of a thermistor device utilizing a polymeric substrate can be lower than 300° C., the manufacturing process is easier, less energy is consumed, and production cost is lowered. Consequently, its application becomes more and more popular as time goes on.
The conductive crystallized polymeric composite material filled with carbon black is under a low resistance status at a room temperature due to its characteristics of the positive temperature coefficient thermistor. When a current flowing through the conductive crystallized polymeric composite material filled with carbon black is too large, and the temperature of the conductive crystallized polymeric composite material filled with carbon black reaches the melting point of polyethylene, volumes of resin in the conductive crystallized polymeric composite material filled with carbon black expand to an extent that makes the conductive stuffing materials in the conductive crystallized polymeric composite material filled with carbon black break down from a continuous status to a discontinuous status. Thus, the resistance of the conductive crystallized polymeric composite material filled with carbon black will rise rapidly, and plaques made of conductive crystallized polymeric composite material filled with carbon black will break the current accordingly. Therefore, plaques made of conductive crystallized polymeric composite material filled with carbon black can be applied to the multi-layer circuit laminated structure for an over-current protection device and a temperature switch device.
However, the interfacial adhesion strength of the plaques made of the metal foil and conductive crystallized polymeric composite material filled with carbon black is not good enough after the thermal laminating process. The adhesion of conductive crystallized polymeric composite material filled with carbon black and metal foil is achieved by a resin inside the conductive crystallized polymeric composite material filled with carbon black. Since the resin tends to flow as the temperature increases, carbon black will fill into the metal electrodes surface of metal laminated material and space between the conductive carbon particles of conductive polymeric composite material. However, the carbon black cannot fully contact with the metal electrodes of the metal laminated plaque, and thus increases the interfacial resistance between metal laminated plaque and the conductive crystallized polymeric composite material plaque filled with carbon black. Moreover, when a laminated structure of the multi-layer circuit is used to fabricate an over-current protection device or a temperature switch device, it has to face various kinds of regular or irregular temperature variation. This leads to problems of adhesions between the electrodes of the metal laminated plaque and the conductive crystallized polymeric composite material plaque filled with carbon black.
To solve the problems of adhesion strength and interfacial resistance, U.S. Pat. Nos. 4,689,475 and 4,800,253 utilize electroplating technique for forming a rough surface with metal nodular protrusions on the surface of metal foil to increase the adhesion strength of the metal electrodes and the conductive crystallized polymeric composite material laminate filled with carbon black.
However, the techniques disclosed by theses patents use carbon black to be directly wedged to metal nodular protrusions and the geometric shapes of carbon black and metal nodular protrusions are different, therefore the contact density is not very good. Meanwhile, mobility of resin on the surface of carbon black is not good between carbon black and metal, the resin can only be adhered to the surface of the metal and thus, increase the interfacial resistance and affect its function.
Furthermore, a known fabrication method of a thermistor is to make the conductive crystallized polymeric composite material filled with carbon black adhere to a foil, such as a copper foil or nickel foil. The method is subjected to the foil material to proceed with a continuous electroplating process for a whole roll of foil, so that the fabrication method is limited.